1. Field of the Invention
The present invention relates generally to a communication system using an orthogonal frequency division multiple access (OFDMA) scheme. More particularly, the present invention relates to an apparatus and a method for transmitting a sub-channel signal while minimizing interference between adjacent cells.
2. Description of the Related Art
Recently, various studies and research have been performed for 4th generation (4G) communication systems in order to provide subscribers with services having superior quality of service (QoS) at a higher transmission rate. In particular, many studies are being performed with the 4 G communication systems in order to provide subscribers with high speed services by ensuring mobility and QoS to wireless local area network (LAN) communication systems and wireless metropolitan area network (MAN) communication systems, which can provide services at a relatively high rate.
In order to support a broadband transmission network for a physical channel of the wireless MAN communication system, an institute of electrical and electronics engineers (IEEE) 802.16a communication system using an orthogonal frequency division multiplexing (OFDM) scheme and an OFDMA scheme has been suggested. According to the IEEE 802.16a communication system, the OFDM/OFDMA schemes are applied to the wireless MAN system to transmit a physical channel signal using a plurality of sub-carriers with a high transmission rate.
The IEEE 802.16a communication system is based on a single cell structure without taking mobility of a subscriber station (SS) into consideration. Additionally, an IEEE 802.16e communication system, which takes mobility of the SS into consideration based on the IEEE 802.16a communication system, has been suggested. The IEEE 802.16e communication system considers the mobility of the SS under a multi-cell environment. In order to permit the mobility of the SS under the multi-cell environment, operational relationship between the SS and a base station (BS) must be changed. Accordingly, studies have been performed with a handover of the SS in order to support the mobility of the SS under a multi-cell structure. Herein, the SS having the mobility is called a mobile subscriber station (MSS).
FIG. 1 is a schematic view illustrating a conventional IEEE 802.16e communication system. Referring to FIG. 1, the conventional IEEE 802.16e communication system has a multi-cell structure including a cell 100 and a cell 150. The conventional IEEE 802.16e communication system includes a BS 110 for managing the cell 100, a BS 140 for managing the cell 150, and a plurality of MSSs 111, 113, 130, 151, and 153. The BSs 110 and 140 communicate with the MSSs 111, 113, 130, 151, and 153 using the OFDM/OFDMA scheme.
The conventional IEEE 802.16e communication system performs an inverse fast Fourier transform (IFFT). For example, the conventional IEEE 802.16e communication system uses 1702 sub-carriers. Among the 1702 sub-carriers, 166 sub-carriers are used as pilot sub-carriers and 1536 sub-carriers are used as data sub-carriers. In addition, the 1536 sub-carriers are divided into 32 sub-channels including 48 sub-carriers, respectively. The sub-channels are allocated to the MSSs according to the state of the system. Herein, the sub-channel signifies a channel including at least one sub-carrier. For example, 48 sub-carriers may form one sub-channel.
The sub-channel can be formed through two schemes in the conventional IEEE 802.16e communication system.
According to the first scheme, the sub-carriers forming the sub-channels are dispersed over all frequency bands of the sub-carriers. In particular, sub-carriers are dispersed over the entire frequency band of the data sub-carriers, thereby obtaining a frequency diversity gain.
According to the second scheme, the sub-carriers forming the sub-channels are aligned in the form of adjacent sub-carriers without being dispersed over all frequency bands of the sub-carriers.
If the sub-channels are formed according to the second scheme, adjacent cells may use the same sub-channel in the same unit time slot. Herein, the same sub-channel signifies the sub-channels including the sub-carriers having the same frequency band. That is, as described with reference to FIG. 1, two adjacent cells (cells 100 and 150) may use the same sub-channel in the same unit time slot.
More specifically, if cells 100 and 150 select the same sub-channel and the same modulation and coding scheme (MCS) is applied to the same sub-channel, the MSS 130, which is located in a cell boundary area, can receive the signal from the BS 110, and also from the BS 140, if the signal has high strength. For example, if the signal has a high carrier to interference and noise ratio (CINR), the MSS 130 receives the signal and demodulates the signal into information data.
If the conventional IEEE 802.16e communication system having a frequency reuse factor of 1 forms the sub-channels according to the second scheme, the sub-channels of the cells forming the conventional IEEE 802.16e communication system have the same frequency band. If the same MCS is applied to the sub-channels of the cells, the MSS located in the cell boundary area can receive the sub-channel signals not only from the BS of the MSS, but also from other BS. As a result, the MSS may receive the sub-channel signal having the higher interference component. Accordingly, it is necessary to provide an apparatus and a method for transceiving the sub-channel signal, while minimizing interference between adjacent cells.